1. Field of the Invention
This invention relates to a method for providing intimate contact between a gas and a liquid to promote their mixing for a variety of applications. More particularly, this invention relates to the use of a hollow fiber contactor for providing intimate contact between a gas and a liquid to promote their mixing. The method of this invention is particularly suitable for removing hydrogen sulfide (H2S) from gas streams.
2. Description of Related Art
Hydrogen sulfide is a contaminant in much of the natural gas produced in the United States and worldwide. It is a poisonous and corrosive gas that must be removed prior to bringing the gas to the customer. The acceptable level for transportation and use is generally less than 4 ppm by volume (4 ppmv). At low concentration levels generally less than about 100 ppmv, but depending upon the total flow so as to address total quantities of hydrogen sulfide of up to a few hundred pounds per day, various scavenger processes are employed, including flowing the contaminated gas through vessels full of either solid adsorbents or liquid solvents, or the injection of liquid solvents into the gas stream. In the latter case, various means are employed to provide gas-liquid interface area, because the reaction takes place at the liquid surface. Long lengths of pipe are required to provide sufficient time for the hydrogen sulfide to react with the injected solvents in the gas stream. These pipes add to the expense of hydrogen sulfide removal, especially in situations where size and weight are important, such as offshore production. In addition, as many existing installations do not have adequate piping lengths to effect the reduction of the hydrogen sulfide to the desired specification by scavenger, or have pipe of such diameter that contact with the scavenger is not effective, a method to improve the contact of scavenger and gas-borne hydrogen sulfide is desirable.
This issue is addressed in part, for example, by U.S. Pat. No. 6,136,282, which teaches a method of hydrogen sulfide removal from natural gas employing a high surface area contactor for providing intimate contact between the solvent and gas in which the flow of gas is divided into a plurality of smaller, parallel streams or pipes rather than one large pipe. While the method provides for increased contact between the scavenger and the gas-borne hydrogen sulfide compared to earlier methods and systems, and while the amount of pipe and space required to carry out the method are reduced compared to earlier methods and systems, it nevertheless remains a desirable objective to further reduce the amount of pipe and space required to remove the hydrogen sulfide from natural gas. This is achievable by further increasing the contactor surface area.
A number of prior patents teach hollow fiber membranes for fluid treatment. U.S. Pat. No. 3,956,112 teaches hollow fiber non-porous membrane solvent extraction by passing a first liquid solvent through lumen of hollow fiber membranes and a solute in a second solvent, which is immiscible with the first solvent, through the space surrounding the hollow fiber membranes, the solute diffusing across the solvent swollen membrane to the first solvent without intermixing the two liquids. U.S. Pat. No. 4,268,279 teaches microporous hollow fibers with a liquid in the lumen and a fluid outside the fiber allowing gaseous components to transfer through the microporous fiber to the inside or outside of the fiber. U.S. Pat. No. 6,228,145 B1 teaches the use of membrane gas/liquid contactors for removing and preventing discharge of carbon dioxide from combustion gases and natural gas from installations for production of oil and/or gas, wherein the combustion gas is passed to an absorber containing a solvent, where carbon dioxide is absorbed in the solvent, and the resulting purified combustion gas, substantially free of carbon dioxide, is discharged into the atmosphere. The carbon dioxide-rich solvent is passed into a desorber where the carbon dioxide is removed from the solvent and the substantially carbon dioxide-free solvent is recycled to the absorber. The membrane gas/liquid contactors are employed in both the absorber and the desorber. U.S. Pat. No. 6,355,092 teaches an apparatus for performing membrane gas/liquid absorption at elevated pressure comprising a pressure vessel in which a membrane unit is provided for separate feed-through of the gas phase and the liquid phase in such a way that exchange of components to be absorbed can take place between the gas phase and the liquid phase, the flow direction of the gas phase through the absorber being essentially perpendicular to the flow direction of the liquid phase through the absorber. However, neither the '145 patent nor the '092 patent disclose mixing of, or contact between, the liquid and the gas.